This invention relates to electronic radio receivers and more particularly to automatic direction finding equipment utilizing a microprocessor implementation.
Direction finding equipment has been utilized in a variety of implementations to provide a bearing from the receiving station to the transmitting station since the advent of widespread use of radio. The original equipments utilized a movable antenna loop which would be manually rotated by the operator to detect the strongest signal. Thereafter the angle of the loop position provides a bearing or reciprocal bearing to the transmitting station. The sense antenna is utilized to eliminate ambiguity when a question may arise as to whether the bearing or its reciprocal is the appropriate angle. This sense and loop antenna combination develops a cardioid pattern, is well known in the art, and provides a single null from which the operator can readily derive the appropriate bearing information.
Subsequent developments in automatic direction finding equipment eliminated the need for rotating antennas with the use of goniometers (RF resolvers) which allowed the use of a fixed loop antenna, in combination with a smaller rotating loop facsimile within the receiver proper. This facilitated the application of ADF equipment to aircraft by eliminating the requirement for moving external antennas.
Subsequently, the need for the rotating goniometer was eliminated by the use of phase comparison of a reference signal, usually 90 Hz, with that of the received station after being modulated by the loop switching frequency sine and cosine signals in the balanced modulator, as is known in the art, to determine the relative bearing to the transmitter. The bearing to the station, .theta., may then be formed by applying the DC sin .theta. and DC cos .theta. signals to a sin/cos RMI or by finding .theta. from: EQU .theta.=tan.sup.-1 (DC sin .theta.)/(DC cos .theta.)
This system has the advantages of entirely eliminating the need for any movement in the receiver or its antennas and can be implemented in relatively simple hardware or with the use of microprocessor and associated radio equipment.
This phase comparison method, although useful, has some inherent disadvantages in that the 90 Hz modulation signal will be readily detected by the operator should a high sense loop ratio or comparison signal be desired to provide a higher comparison ratio. This is particularly noticeable when the transmitting station is a commercial broadcast station providing an audio program, and the operator desires to listen to the content of the audio broadcast, for example to ascertain the identity and therefore location of the transmitting station. While listening to the audio content, the phase comparison system may frequently develop a relatively strong audio distortion in the phase comparison process. If so, the audio signal will tend to be garbled by the 90 Hz signal. This problem is highlighted by the need in the phase comparison system for relatively high sense loop ratios because the system tends to be relatively easily disturbed by a stronger interferring transmitter in close angular proximity to a weaker targeted transmitting station. To merely increase the sense loop ratio to overcome this difficulty can result in a highly distorted audio output.
Additionally, phase measuring techniques are susceptible to envelope delay of the modulation through the receiver or phase shift of the bearing signal following demodulation. Any envelope delay or phase shift of the bearing signal causes a bearing error and means must be provided to calibrate the system and minimize bearing variations over environmental conditions, particularly temperature variations.